If one single piece of code has improved Del Harvey's life, it's got to be the addition of text shortcuts to the iPhone software. When iOS 5 introduced text expansion – type a few predetermined letters, and it will expand them to a whole phrase or sentence – the task of replying to people who are having problems with their Twitter account eased at once.

Harvey is senior director of Twitter's "Trust & Safety" team, and if you look at her Twitter account – which has 313,000 followers – you'll find that a phrase that appears again and again is "You'd want to file a [support] ticket via bit.ly/gottaproblem", often followed by "Tickets are answered in the order received". She used to type those by hand, with the incumbent risk of error; now it's just a couple of shortcuts.

For that, Harvey gives quiet thanks; she answers a lot of those in a typical week. Having shortcuts allows her to get on with the bigger challenges at Twitter, where she used to be the lone person in charge of the service's spam and other security challenges when she joined in 2008. (She survived blocking the founders' accounts as spam in her first month on the job.)

Now, the team is much bigger, but so are the challenges. Twitter spam – once one of her main focuses – hasn't been beaten, but it hasn't grown proportionally to the number of users or tweets. Instead, her department tends to focus more on the broad issues: ensuring transparency, getting malware off the system, and more recently dealing with wider issues such as removing links to child abuse pictures. "Trust & Safety" means, essentially, making sure that people can trust Twitter when they log on to it even as the service grows at breakneck speed.

Harvey's LinkedIn profile gives some clues to her dry – verging on astringent – sense of humour. "My experience and goals can perhaps best be summarised by Vanilla Ice," it says. "If there was a problem, yo I'll solve it / check out the hook while my DJ revolves it." She continues: "I do things, where 'things' is an undefined set within the broader range of 'somebody ought to take care of those …'"

The "things" include everything from Twitter's terms of service to privacy policies; "I ensure user trust and protect user rights and mitigate legal risks," she explains.

"Biz [Stone] and Ev [Williams] thought there wouldn't be a problem with spam because you can choose who you follow. Wrong," she notes. But she also notes that "anecdotally, [spam] seems small. But, you know, we work on getting rid of one and another pops up. The anti-spam engineers are doing some really cool work" – which she won't elucidate on.

The number of users and more importantly the number of tweets that go through the service has increased enormously. It took three years and a couple of months to reach the first 1bn tweets sent. Now, 2bn are sent every working week. A billion per day is probably in sight within a year.

Twitter has grown too. It recently moved to new premises on San Francisco's Market Street, an area where the homeless and the unemployed play chess on the pavement alongside pawn shops that offer you a good deal if you have some diamonds to trade in. When I mention that I'd felt I stood out slightly from most of Market Street's denizens because I was wearing a suit jacket, Harvey remarks drolly "I think you'd get less attention on Market Street if you walked along it in the nude than if you wore a suit jacket." She pauses. "Literally," she adds.

Twitter's new offices, though, are six floors and a world away from Market Street. Located in a former furnishings warehouse recently redeveloped by a local property speculator, it is vast yet also well-lit; the centrepiece is the canteen, which has an outside space with its own lawn. As befits a Silicon Valley company (in name at least), the range of food is plentiful and the coffee seems never-ending.

Around the building, short slogans – "Simplicity" or "Defend and respect the user's voice" – appear in rotation on backlit screens, each intended to remind people of the company's aims. Dick Costolo, the chief executive since a boardroom shakeup in October 2010, summed up Twitter's mission statement in 2011 as "To instantly connect people everywhere to what's most important to them".

It hasn't stuck just to words, of course; Twitter has added pictures, music and acquired Vine, which has enabled six-second videos to become an art form. And of course before that it created its own link shortener, t.co, through which every single link posted to Twitter; any found to be malware or spam won't resolve, and trying to post or repost it will fail.

And there are other things where the Trust & Safety team has had an important effect on how you experience Twitter, she says. "In March 2012 we launched 'POC' – point of contact – meaning that every product manager has a point of contact in Trust & Safety who they can get in touch with, so if they're proposing something new there's somebody from our team following it. So, for example, there's an option to have an interstitial [notice] before you view sensitive content. That's there because we were involved in its development."

"If you're launching image embedding on Twitter, then you might not think about how to stop people seeing image embeds. Why would you? You're trying to put images in there, not stop them being seen." But if an image might be offensive, you don't want to present that to the user without warning. So you have to opt in – either one-off, or (via a setting) permanently – to seeing images without getting a warning.

Next are the wider problems around images. One of the projects Harvey is most keen on is called "PhotoDNA" – a Microsoft-developed technology which will produce a unique "fingerprint" of every picture put on Twitter's service and be able to identify images of child exploitation. It's already used on Microsoft's Hotmail (now Outlook), Skydrive and Bing services, and Facebook adopted it in 2011.

It's not the only thing, but for Harvey it's a big thing. Before she joined Twitter, she worked for an organisation called "Perverted Justice", set up in 2002, which used to organise stings to catch sexual predators. In 2006, the US TV network NBC gave it $802,000 for helping it on its TV series "To Catch a Predator". (Harvey was the "secretary" at the organisation, on a six-figure salary.) The organisation helped in 535 convictions around the US, and Harvey received a commendation from a Californian police department for her work on one sting.

So the concentration around child abuse, and getting such images off the web, chimes with her. "There's a lot that's happening concurrently, on a lot of different fronts. One of the things that's exciting is that we're working on implementing Photo DNA which lets you match images hashes for nude child porn images. We're working with a bunch of the other companies in this space around a bunch of stuff in this space, that's exciting."

She explains: "Given the length of time that I've been working on issues tie to child exploitation, it's really fantastic that we're making progress on getting that in place. And it's good that others in the industry are working on it or implementing it, because it's not an area of competition [between companies]."

But with Photo DNA, Twitter faces what Harvey calls "the scale/speed problem". "We first fell into that problem with [link shortener] t.co," she explains. "We did think of putting a sort of wrapper around the pictures." The simple challenge is how to check so many pictures when the service sees about 2bn tweets every working week.

So does Photo DNA have a timescale for its release? "It's a little bit 'ready when it's ready'. Partly because, with scale, it becomes so incredibly complicated to think how the implementation will work," Harvey says. "You think 'we'll just delete the image' – but is it hosted on a CDN [content delivery network, which caches frequently-viewed content closer to its destination] somewhere? Then how do you make sure that it gets flushed? These aren't just standard images, but images you want removed as soon as possible. How do you flush those? You can't just have a queue of requests, because what happens if there's a sudden surge and then a backlog of images to be flushed? And you're like really? Really, does it have to be this complicated? And the answer is yes, it really does have to be that complicated."

But she adds that implementing Photo DNA "is something that Product has committed to. They have folks figuring out how to best implement it, and I'm hoping that it's a this-year thing. But I can't swear to it."

Then there are other contentious issues – such as the suggestion by some Twitter users inside Turkey that the government there is filtering out tweets which oppose its government.

"I've heard that there's been occasional access issues, and there was a URL that involved a homomorphic character" – she grins – "I just learned that word recently, so I had to trot it out. Homomorphic, that's where it's a Cyrillic alphabet character so it looks like an i but it's not; it's missing the dot, and it's like 'Twitter; minus that i. That URL doesn't lead you here [to twitter.com] – when you click on it, it leads you to – I think – a [Turkish] government website. So there could certainly be people who have been" – she slows down while she picks her words – " …. bemused by that. And then there's always the possibility of general blocking."

Having introduced two-factor authentication (a long called-for feature) in May, the company is still working on other ways to protect users' accounts while also helping them to use them when they have trouble. "Things like password resets – people have the problem where they can't remember what email they registered with, or they don't have control of that email any more." (The latter being the way that the Syrian Electronic Army managed to put out tweets on Twitter accounts belonging to the Associated Press, Financial Times, BBC and, yes, the Guardian.) "Maybe you'd have a protection scheme where you have to put in some personal information to reset it," she suggests.

What about the fallout from the Guardian's NSA revelations – including those about Prism (where Twitter was notably absent from the companies to which the NSA was claiming "direct access")? Harvey shrugs. "Transparency is the way to go," she says. "I honestly hope that that is the way it does go. But it's a good conversation to have."